The present invention relates to electronic watthour metering and, more particularly, to techniques for overcoming offset voltages at an input of an amplifier serving as part of an active-feedback element in a current transformer feeding an electronic watthour meter.
One type of electronic watthour meter disclosed, for example, in U.S. Pat. Nos. 3,947,763 and 3,955,138 employs a multiplier using a pulse-width-modulated signal having pulse widths ratios related to an instantaneous value of one of line voltage or load current to modulate the instantaneous value of the other thereof. The resulting modulated product signal is proportional to the product of line voltage and line current; that is, to the instantaneous power consumption of the load. The product signal is integrated to produce a sawtooth waveform which triggers an output pulse when its amplitude attains a predetermined value. Upon the production of the output pulse, the direction of integration is reversed until an opposite threshold is attained. Each cycle of the output pulse indicates the consumption of a predetermined quantum of electric energy, conventionally measured in units of watthours or kilowatthours.
The load current is typically many times the value of a current signal appropriate for use in an electronic watthour metering device. In some systems, the load current is as much as 10,000 times larger than the desired current signal. It is conventional to employ a current transformer wherein a small number of turns (for example, one or two) about a toroidal core serve as a primary current-transformer winding carrying the load current. A secondary winding of many turns has induced in it a current proportional to the load current but reduced by the primary-to-secondary turns ratio of the current transformer.
Current transformers are prone to core saturation in the presence of large primary currents. Core saturation conventionally is avoided by using large cores and making the cores of high-quality materials. Both large size and high-quality materials invoke high cost.
One solution for core saturation disclosed, for example, in U.S. patent application Ser. No. 944,028, filed 12/22/86, and the references cited therein, includes providing a feedback winding on the core carrying an inverse current signal just sufficient to maintain the core flux near zero. Limiting the core flux near zero permits using smaller cores and cheaper core materials. As the load current increases, the inverse current signal also increases just enough to maintain the core flux near zero, whereby all levels of load current can be accommodated without experiencing core saturation of the current transformer. The inverse current may, itself, be employed as the output signal from the current transformer.
The active feedback employed in the above-referenced patent application is produced by an operational amplifier receiving the output of the secondary winding of the current transformer. The conventional high gain of an operational amplifier produces an inverse current easily capable of maintaining near zero flux in the core. The high gain of the operational amplifier, however, leads to a further complication. That is, coupling between the feedback winding and the secondary winding of the current transformer is only for alternating current (AC). There is no DC feedback coupling to the input of the operational amplifier. Thus, DC offset voltages of, for example, a fraction of a milliampere may appear at the input of the operational amplifier. Operational amplifiers conventionally have DC gains on the order of several million. As a consequence, any offset voltage, even a fraction of a milliampere, at the input of the operational amplifier is capable of driving the operational amplifier to saturation.
One technique for offset-voltage compensation is disclosed in U.S. patent application Ser. No. 70,794, filed 7/7/87. This device momentarily short-circuits the input of an operational amplifier and stores a sample of any output voltage as a measure of the offset voltage. During subsequent operation, the sample is applied to the input of the operational amplifier along with the input signal. This technique appears to have little value in solving DC offset problems in an operational amplifier producing a negative current signal for maintaining the flux in a current transformer core near zero.
U.S. Pat. No. 4,066,960 discloses compensating for offset voltages by integrating the product signal first in one direction and then in the opposite direction during the half cycles of the output pulse of an electronic watthour meter. Any influence of offset voltages on the length of a half cycle of the output signal is exactly compensated by an equal and opposite effect on the length of the adjacent half cycle. Although useful for avoiding errors in an integrator, this technique is not helpful in the present application since any integration taking place is well downstream of the point at which offset voltage acts.